According to the "Glossary of Terms Used in the Pressure Sensitive Tape Industry," a pressure sensitive adhesive (PSA) is a material which is aggressively and permanently tacky, adheres without the need of more than finger pressure, exerts a strong holding force, and has sufficient cohesiveness and elasticity that it can be removed from substrates without leaving a residue.
Low T.sub.g acrylic copolymers are being used as PSAs. Such copolymers have usually been prepared by emulsion polymerization of acrylic monomer mixtures through free radical initiation. Copolymers made by this process usually have molecular weight distributions, M.sub.w /M.sub.n, in the range of 2-4 as measured by gel permeation chromatography (GPC). While the resulting adhesive is often satisfactory to obtain certain properties, such as tackiness and peel strength on substrates such as polyester or stainless steel, certain properties are generally found to be deficient, particularly the holding power on these same substrates. Previously, increasing the peel strength of acrylic copolymers to a value greater than 2.0 pounds per inch generally caused the holding power (1 kg over one inch square on stainless steel) to diminish to below 40 hours. Attempts to increase the holding power to over 40 hours, however, resulted in a peel strength lower than 2.0 pounds per inch. In practical applications, it is desirable to maintain a high holding power while improving the peel strength, particularly when the PSA is to be used to prepare an adhesive tape.
Various polymerization techniques have been attempted to prepare satisfactory acrylic polymer emulsions. For example, in Japanese Patent J-51125472, a petroleum resin emulsion is obtained by polymerizing vinyl monomers in the presence of petroleum resins having softening points of from 40.degree. to 160.degree. C., an average molecular weight of 300 to 3000, and an acid value and saponification value of less than 1. The monomers include, for example, alkyl (meth)acrylates, vinyl acetates, vinyl chlorides, styrene, acrylonitrile, and acrylic acid. The emulsified mixture is then reacted in an emulsion polymerization reaction to form a shelf-stable emulsion adhesive. The resin emulsion produced is described as having fine particle sizes and ample stability and when cured, the films produced have excellent water resistance and gloss.
Also, U.S. Pat. No. 4,645,711 to Winslow et al. describes the incorporation of hydrocarbon resins from many sources, i.e., from hydrogenated resin esters, polyterpene, polymerized alkyl styrene, and polymerized petroleum-derived monomer resins, into PSA tape compositions where the adhesive is a polymerized acrylic emulsion. The patent describes physically mixing the resin with the polymer emulsion.
Numerous approaches have been used to produce resin emulsions. One approach is to dissolve the resin in a hydrocarbon solvent, combine the resin solution and water to form an emulsion, and strip the solvent. Invariably some residual hydrocarbon solvent remains in the finished emulsion which is undesirable in certain applications. This has led to the development of solvent-free dispersions (see U.S. Pat. No. 2,809,948) and emulsions (see U.S. Pat. No. 3,377,298) of petroleum resins. In both of these formulations, ionic emulsifiers have been utilized. In the former, a mixture of cationic and non-ionic surface active agents is used to achieve a resin emulsion, and in the latter an ionic surfactant is used in combination with an aqueous gel of a swelling earth to produce an emulsion paste of a petroleum resin. These resin emulsions have been commercially used to tackify natural rubber, carboxylated styrene-butadiene and acrylic latexes for many adhesive applications.
The art is also replete with various polymerization techniques intended to improve other properties of polymers obtained through either redox initiation or temperature ramping during polymerization. For example, in U.S. Pat. No. 3,700,456 to Smith, there is described the preparation of a photographic silver halide emulsion by emulsion polymerization of acrylate and other unsaturated monomers with a redox initiator wherein the oxidant is bromate or chlorate and the reductant is bisulfate. Other examples of bromate-sulfite, -bisulfite and -metabisulfite polymerization redox initiators are found in U.S. Pat. No. 3,929,751 to Gershberg (polyacrylamide); U.S. Pat. No. 4,748,217 to Malhotra (polytetrafluoroethylene); and U.S. Pat. No. 4,739,008 to Robinson et al. (water-in-oil biphase polymerization).
In U.S. Pat. No. 4,588,791 to Bieringer et al., vinyl chloride was suspension polymerized with an ascending temperature control to produce a polyvinyl chloride said to have a broad molecular weight distribution. The polyvinyl chloride product was said to exhibit improved flowability for injection molding and to produce injection molded articles having smooth, transparent surfaces.
In U.S. Pat. No. 4,536,556 to Tijssen et al., there is described the emulsion polymerization of .alpha.-methylstyreneacrylonitrile copolymers with a free radical initiator and a chain transfer agent wherein the temperature increases over the course of the polymerization. This technique is said to maintain monomer conversion rates at a maximum and to improve heat resistance.
Accordingly, there still remains a need for improvements in the procedures used to prepare an acrylic copolymer in the form of a latex-like dispersion for application as a PSA having increased peel strength without serious decrease in shear. As mentioned above, there has generally been a decrease in shear properties associated with modifications made in an effort to obtain an increase of peel strength and tackiness. There is a need to improve the peel strength of PSAs while yet maintaining high shear properties and thus obviating the decline in shear usually resulting from variations intended to increase peel strength.